1. Field of the Invention
This invention relates to apparatus for use in pouring molten metal from a teeming vessel through a shroud tube into a receiving vessel such as the mold of a continuous caster, and more particularly to an improved apparatus for changing the shroud tube used in such apparatus and for supporting the shroud tube in a manner to maintain a more positive seal with the nozzle or tube holder of a sliding gate throttling valve used on a teeming vessel.
2. Description of the Prior Art
In the pouring of molten metal from a bottom pour teeming vessel such as a tundish used in the continuous casting of steel, it is conventional practice to control the flow of the liquid metal from the tundish by use of a sliding gate throttling valve. Metal from the throttling valve flows through a pouring tube, or shroud tube, constructed of a high temperature resistant ceramic or other material and having one or more outlets submerged in the liquid metal contained in a receiving vessel such as the continuous caster mold. A throttling valve and shroud tube assembly of the type commonly used in the continuous casting of steel is illustrated and described in U.S. Pat. No. 4,415,103, the disclosure of which is incorporated herein by reference and this invention will be described with reference to the teeming of molten steel from a tundish through a valve of this general type in a continuous casting operation, it being understood that the invention may be used in conjunction with other valve mechanisms and in other metal teeming operations.
In the continuous casting of steel using a throttling valve and shroud tube, it is critical that a good seal be maintained between the shroud tube and the throttling valve to avoid exposure of the molten metal stream to the atmosphere as it passes through the assembly. The aspiration effect of the flowing steel stream can readily draw sufficient air through eve a relatively small opening to cause substantial increase in the oxygen content of the steel so that it may be necessary to downgrade the cast product and sell it at a reduced price.
The high temperatures to which the shroud tube is subjected in a continuous steel casting operation, combined with the errosive effect of the molten steel flowing through the shroud tube and the stresses and errosive effects produced by the continuous movement of the caster mold into which the lower end of the shroud tube projects makes it necessary to frequently change the shroud tube. Shroud tubes formed from ceramic material of the type commonly used may have a life expectancy of from 1/4 to 1/2 that of the mating tube holder assembly in the throttle valve. Nevertheless, in view of the criticality of maintaining a good seal between the two parts and to minimize the time during which the metal flow is interrupted during changing of a shroud tube, it is conventional practice to preassemble a new shroud tube and tube holder which assembly is then installed as a unit. While this practice makes it possible to remove and replace a spent shroud tube relatively quickly, it requires substantial time in assembling the shroud tube and tube holder in a manner to provide the necessary airtight seal and requires a substantial sacrifice in ceramics by discarding a tube holder, which otherwise might have substantial useful life remaining, each time a shroud tube is changed.
Numerous devices have been developed for handling and manipulating shroud tubes of the type employed to enclose a stream of molten metal flowing from the bottom pour discharge nozzle of a ladle into a casting tundish. In such an operation full ladles are positioned above the tundish, emptied and removed in succession, making it necessary to change the shroud tube upon each ladle change. In such operation, positioning the shroud tube normally requires careful alignment and manipulation, after which the shroud tube may be releasably secured to the support structure of the valve as disclosed, for example, in U.S. Pat. No. 4,316,561.
Shroud tube positioning and manipulating apparatus for use in connection with a teeming ladle is also known which continuously supports the tube and retains it in position in contact with the outlet nozzle during teeming. Such devices are normally supported by rigid structure independent of the ladle, making it necessary to support the shroud tube with a biasing force which permits limited vertical and horizontal movement with corresponding movement of the heavy ladle during the teeming operation. One such device is disclosed, for example, in U.S. Pat. No. 4,550,867 which employs a fluid cylinder in connection with an articulated arm supported on a rigid frame structure for maintaining a continuous biasing force between the shroud tube and gate valve while permitting movement of the shroud tube with the ladle. The use of a counterbalancing weight is also known for applying a biasing force in such shroud tube support.
The use of a counterweight to apply a biasing force to a shroud tube used in connection with a casting tundish throttle valve is also known and employed in a commercial slide gate throttling valve manufactured by Interstop Corp. as shown in Iron and Steel Metallurgy, May 1987, pg. 30. In this construction, a horizontal track arrangement mounted on the flow control valve supports rollers adjacent a yoke on one end of an elongated arm for supporting a shroud tube, and an outwardly projecting eyelet on the opposite end of the arm for supporting a counterweight. Such arrangements, however, are inherently difficult to manipulate in that the counterweight must be lifted and moved with the arm upon retracting the apparatus, and removal of the hot spent shroud tube and installation of a second tube is inconvenient and time consuming. Further, as erosion of the shroud tube takes place counterbalancing force from the fixed counterweight will vary.
In order to overcome the disadvantages of the prior art, it is a primary object of the present invention to provide an improved shroud tube supporting and changing apparatus for use in the bottom pour teeming of molten metal.
Another object is to provide such a improved apparatus which enables very rapid changing of the shroud tube and which provides a more positive air seal between the shroud tube and throttling valve.
Another object is to provide such an apparatus which is mounted directly on and supported by the teeming vessel and which is operable to accurately, easily and reliably position a shroud tube relative to the throttling valve.